What's really interesting to me right now is identity, aggregation, and real time processing on the web:
identity - it is so nice to have a single identity that works across different web sites. Specifically, being able to log into another web site with my facebook account is great. I don't have to do any email validation, and when I'm done using the site I can ask facebook to revoke their access. All the issues dealing with privacy and protocol to share this information are really interesting. Things to fully understand for this: OpenID, OpenAuth, Facebook authentication.
aggregation - once you've got a single identity, you should be able to pull in all your data into one interface or archive. When I send a video to Erin, I want it archived somewhere so I can find it later.
real time processing - I want to see things on the web as they're happening. The facebook action stream is great - there should be some open protocol for this. see actionstrea.ms.
